Whilst the following discussion is with respect to hip replacement surgery, a person skilled in the art will appreciate that the present invention is not limited to this particular field of use and may be adapted to use with any bone structure or various types of surgery.
Hip replacement surgery involves the use of a prosthetic cup (acetabular cup) or a prosthetic ball (femoral stems) or both to restore the ball and cup joint functionality of the hip. The ball and cup joint enables the hip to rotate in different directions to various degrees (in contrast to the relatively limited rotation of a knee joint).
In 2001, approximately 165,000 total hip replacements were performed, according to data from the American Academy of Orthopaedic Surgeons, using figures from the National Centre for Health Statistics. Historically, hip replacement (arthroplasty) surgery required up to a 40 cm (7 to 12 inches) curved incision to provide sufficient access for the surgeon to manually access and manipulate the hip and femur. A prosthetic cup was attached to the hip socket or the head of the femur removed and replaced with a prosthetic ball, or both.
After the incision is made, the ligaments and muscles are separated to allow the surgeon access to the bones of the hip joint. It is typically this part of the surgery that makes the ligaments and muscles somewhat weak after surgery. Until they heal, which often takes about a month to six weeks, the patient must follow special hip precautions to prevent dislocation of the new hip joint.
Typical steps in hip replacement surgery include the following:
Removing the Femoral Head: Once the hip joint is entered, the femoral head is dislocated from the acetabulum. Then the femoral head is removed by cutting through the femoral neck with a power saw.
Reaming the Acetabulum: After the femoral head is removed, the cartilage is removed from the acetabulum using a power drill and a special reamer. The reamer forms the bone in a hemispherical shape to exactly fit the metal shell of the acetabular component.
Inserting the Acetabular Component: A trial component, which is an exact duplicate of the patient's hip prosthesis, is used to ensure that the joint received will be the right size and fit. Once the right size and shape is determined for the acetabulum, the acetabular component is inserted into place. In the uncemented variety of artificial hip replacement, the metal shell is simply held in place by the tightness of the fit or with screws to hold the metal shell in place. In the cemented variety, a special epoxy type cement is used to “glue” the acetabular component to the bone.
Preparing the Femoral Canal: To begin replacing the femoral head, special rasps are used to shape and hollow out the femur to the exact shape of the metal stem of the femoral component. Once again, a trial component is used to ensure the correct size and shape. The surgeon will also test the movement of the hip joint.
Inserting the Femoral Stem: Once the size and shape of the canal exactly fit the femoral component, the stem is inserted into the femoral canal. Again, in the uncemented variety of femoral component the stem is held in place by the tightness of the fit into the bone (similar to the friction that holds a nail driven into a hole drilled into wooden board—with a slightly smaller diameter than the nail). In the cemented variety, the femoral canal is rasped to a size slightly larger than the femoral stem. Then the epoxy type cement is used to bond the metal stem to the bone.
Attaching the Femoral Head: The metal ball that replaces the femoral head is attached to the femoral stem.
The Completed Hip Replacement: Before the incision is closed, an x-ray is taken to make sure the new prosthesis is in the correct position.
Such surgery had a number of problems including:                a hospital stay of three days or more, post-operative pain and weeks of rehabilitation;        each cm of incision has a tenfold increase in the risks of blood clotting and infection post surgery; and        the surgeon was reliant on his experience and eye to ensure accurate placement of the cup into the three dimensional hip socket and alignment of the cup with the ball/femur to enable proper function of the joint. Misalignment may lead to post operative complication such as misalignment of the leg, incorrect leg length and/or incorrect soft tissue tension. The long term effects of misaligned prosthetic components can also include accelerated wear of the components, aseptic loosening of the components and potentially early repetition of the surgery.        
Attempts to overcome these problems include:                WO 2003/037192 which discloses a jig (impaction tool) for use in bone surgery and thus enables the use of a smaller incision. For hip replacement surgery, the jig enables the use of a 4 to 7 cm (2 to 3 inch) incision, i.e. keyhole surgery. Other benefits include a shorter stay in hospital, less blood loss, less pain, fewer postoperative dislocations and faster recovery; and        WO 2005/046475 which discloses a gauge to assist the surgeon with accurate placement of a prosthetic when using a jig in keyhole surgery as the surgeon is no longer able to see the fit of the cup into the hip socket or the fit between the ball and cup.        
The gauge provided in WO 2005/046475 has enabled efficient use of the impaction tool of WO 2003/037192. Commercial examples include the NilNav Hip System available from MAC Surgical. However, the gauge only works in two dimensions and there is still a heavy reliance on the surgeon's eye and experience for optimal placement of the cup into the hip.
There is thus a need to further aids to assist the surgeon during surgery.